drivers: bluetooth: da1469x: Fix deferred buffer allocation

If rx buffer allocation has to be deferred to rx_thread, we need to stop
isr reading from mailbox as otherwise rx_thread won't be able to process
other buffers.

Since CMAC2SYS irq is cleared before data is read from mailbox, in case
rx buffer allocation was deferred we also need to trigger irq manually
to make sure all pending data is processed.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>